The invention relates to an anti-buckling coupling device for piping. The invention is more specifically, but not exclusively, designed to realize couplings intended for joining ducts in an aircraft, and more specifically for fuel piping made from double-walled ducts.
In many vehicles, especially aircraft, rigid piping carrying different kinds of fluids is fastened to said vehicle's structure by anchor points. Advantageously the couplings between two ducts of the piping are placed at the location of these anchor points, so that the weight of said couplings is taken up by the structure of the vehicle and not by the piping. Thus, said couplings are linked to the ducts by means of a groove connector, or sphere cylinder, i.e. a connector that combines a swivel connection and a sliding connection along the longitudinal axis of the duct. This connector allows the alignment faults between the ducts forming the piping and the variations in distance between the anchor points to be accommodated; these faults or these variations result from:                tolerances, either in the manufacture of the ducts or in the positioning of the anchor points in the vehicle's structure, and        differential deformations of the piping and the structure when they are subjected to the vehicle's operating stresses.        
These degrees of freedom at the couplings are realized with limited amplitudes so that hermeticity is always guaranteed and the ducts thus assembled cannot become disconnected.
FIG. 1 schematically illustrates a duct assembly relative to the prior state of the art. As FIG. 1A shows, according to this simplified representation, the ducts (101, 102, 103) forming the piping are straight ducts, arranged along a rectilinear alignment (110) of their axes. Each duct comprises a skirt (121, 122) at each of its extremities. These skirts are inserted into couplings (131, 132); these couplings are fixed to a structure, for example the fuselage of an aircraft, by anchor points (191, 192). Each coupling comprises sealing means (155) producing a substantially elastic annular linkage between the skirt (121, 122) of the duct and the interior bore of the couplings (131, 132). The exterior diameter of the skirt (121) is smaller than the interior diameter of the coupling (131) such that outside the contact with the sealing means (155), the skirt (121) has a radial gap (r) with the interior bore of the coupling (131). Thus, in each coupling, the skirt of each duct has a swivel connection with said duct via the annular contact with the sealing means (155) and the elasticity of these means. As FIG. 1B shows, the degrees of liberty introduced by this swivel connection permit a misalignment of the couplings (131, 132) with no damage and no assembly difficulty for the duct (101). The angle of misalignment allowed depends on the flexibility of the sealing means (155), the radial gap (r) between the skirts (121, 122) and the bores of the ducts (131, 132) and the insertion length (L) of the skirt (121, 122) into the bore of the coupling (131, 132). Thus, for the same allowed angle of misalignment, the longer the insertion length (L) of the skirt into the coupling, the greater the radial gap (r) must be.
As FIG. 1C shows, the length of the skirts (121, 122) and their implantation length (L) in the couplings are determined by the connector's ability to accommodate variations in distance (Δd) between the couplings (131, 132). These variations in distances arise from positioning tolerances for the couplings in the structure and elastic variations in distances between the couplings; these elastic variations result from the structure's response to the operating stresses to which it is subjected. The couplings (131, 132) comprise translational movement stoppers (165) to avoid the accumulation of elastic deformations over time leading to one extremity of the duct becoming disconnected from the coupling. Thus, the implantation distance (L) of the skirts (121, 122) in the couplings is determined as follows:                when the two couplings (131, 132) are in their closest relative position (figure C1), the two skirts (121, 122) are in contact with the translational movement stopper means (165);        when the couplings (131, 132) are in their most distant relative position (figure C2), one of the skirts (121) is in contact with the translational movement (165) stopper (165) in its respective coupling (131) while the extremity of the other skirt (122) is at a distance from the translational movement stopper (165) in its respective coupling (132), which distance is equal to the maximum variation (Δd) in distance between the two couplings while always ensuring contact with the sealing means (155).        
Thus, whatever the position of the duct between the two couplings, it cannot become disconnected from one of the couplings and, as a result, the implantation distance of the skirts in each coupling is equal to the maximum potential variation in distance (Δd) between the anchor point (191, 192), increased, where appropriate, by a safety coefficient and a margin to take into account the cumulative influence of the misalignment. The potential variation (Δd) in distance between the couplings has three origins:                i. the manufacturing tolerances of the ducts, the couplings and positioning of the anchor points bearing said couplings;        ii. the elastic variations in distance between the anchor points, as a result of the structure's nominal operating stresses;        iii. and, for some piping, fuel piping in particular, the exceptional variations in distance between the anchor points.        
The exceptional variations in distance occur in situations referred to as crash situations. The goal is to preserve the hermeticity of the piping in these exceptional conditions to prevent the fluid they transport spilling into the vehicle. Thus, a significant lessening of the distance between anchor points can produce a compression of the duct, which is therefore likely to be deformed by buckling and to break. These exceptional distortions of the structure are likely to have high values; thus, taking them into account in the implantation length of the skirts according to the principle of the prior state of the art described above, while ensuring that the accumulation of elastic deformations does not result in a disconnection of the ducts, leads to especially long lengths of skirts and implantation lengths of these skirts in the couplings. In order to preserve the capacity to accommodate misalignments, this increased implantation length requires an increase in the radial gap (r) of the skirts in the couplings. Thus, taking these exceptional constraints into account according to the design principles of the prior state of the art leads to a significant increase in the mass of the piping.